Why does the light have a speed?

The speed of light is constant in all media, including a vacuum. In - for example - a piece of glass light takes longer to traverse it. But this is not becuase the light actually travels slower. It is becuase the photons interact with electons within the atoms of the glass, being absobed and re-emitted. It is travelling in a stop-start kind of way, but while it is moving it still moves at the same speed. Light speed is better thought of as the Universal Constant, rather like other constants of nature, like Planck's Constant, the Gravitational Constant, etc. It just so happens that the velocity of electromagnetic (and other) waves is propogated through space directly proportionally to this constant, with the constant of proportionality being 1, whereas the constant of proportionality in other phenomena is not 1. Just a quirk of our way of defining our SI units and that we choose to work in base 10 mathematics.

One interesting way of thinking about light (as suggested by the author Timothy Ferris) is to note that according to the equations of relativity, objects travelling at the speed of light have their time from the perspective of an external observer slowed down to a halt. This means that if we could place a clock on board a photon and look at the time when the photon sets out, and then look again onced it arrives somewhere distant, the clock will not have ticked once. In some senses the photon's own experience of travel is that it arrives instantaneously. Thus in the same sense, the two points in space (start and finish) are in fact the same point from the perspective of the photon. Have fun with that idea...

Cheers.

Originally posted by d60944
The speed of light is constant in all media, including a vacuum. In - for example - a piece of glass light takes longer to traverse it. But this is not becuase the light actually travels slower. It is becuase the photons interact with electons within the atoms of the glass, being absobed and re-emitted. It is travelling in a stop-start kind of way, but while it is moving it still moves at the same speed.

Not realy, the interaction with the atoms is called diffraction...
The speed of the light is, according to Maxwell equation, related to the local permeability and permitivity...

Thus in the same sense, the two points in space (start and finish) are in fact the same point from the perspective of the photon. Have fun with that idea...

Back to the holografic univers I like it

Originally posted by I_AM_that_I_AM
Light moves slower through the air than it does through vacuum and slower through water than it does through air.

Recently they've even slowed it down to a few mile per hour in labs.

Actually light was stopped completely. Quite exciting news!

Light stopped in its tracks

To stop light altogether, the scientists have utilised a similar but far more powerful effect. The researchers cooled a gas of magnetically trapped sodium atoms to within a few millionths of a degree of absolute zero (-273 deg C).

More ...

The following article also covers much on the topic covered here:

Einstein's relativity theory hits a speed bump

Australian scientists have discovered that light isn't quite as fast as it used to be. But it doesn't mean E=mc2 will be consigned to the dustbin, writes David Wroe.

Much more in article...

Originally posted by Lillo

Originally posted by d60944
The speed of light is constant in all media, including a vacuum. In - for example - a piece of glass light takes longer to traverse it. But this is not becuase the light actually travels slower. It is becuase the photons interact with electons within the atoms of the glass, being absobed and re-emitted. It is travelling in a stop-start kind of way, but while it is moving it still moves at the same speed.

Not realy, the interaction with the atoms is called diffraction...
The speed of the light is, according to Maxwell equation, related to the local permeability and permitivity...

Ooops. Yes. I apologise for my jumping to conclusions. The interactions with the electrons in the substance are not mere absorbsions and re-emissions (diffraction), but the differing electormagnetic permitivity of materials is still caused by interaction with these electrons in a different way:

At the microscale, an electromagnetic wave's phase velocity is slowed in a material because the electric field creates a disturbance in the charges of each atom (primarily the electrons) proportional to the permittivity. The charges will, in general, oscillate slightly out of phase with respect to the driving electric field. The charges thus radiate their own electromagnetic wave that is at the same frequency but with a phase delay. The macroscopic sum of all such contributions in the material is a wave with the same frequency but shorter wavelength than the original, leading to a slowing of the wave's phase velocity.

(en.wikipedia.org...)

Cheers.

[edit on 10-1-2006 by d60944]

Originally posted by Lillo
The speed of light doesn't depend of its frequency: it is constant for all the frequency in given medium(the fastest is in vacuum).

For things traveling faster than light, the theory says it impossible.

But when an object, can dissapear at one place, and at that same moment reappear on a spot let's say 1000000 kilometers removed from the spot it dissapeared, it travelled through time, at a faster speed than light (visual light, like the light from the sun or lamps. right?

Originally posted by alienaddicted
But when an object, can dissapear at one place, and at that same moment reappear on a spot let's say 1000000 kilometers removed from the spot it dissapeared, it travelled through time, at a faster speed than light (visual light, like the light from the sun or lamps. right?

The teleporting of an object has still not been done experimentaly (for what I know), but let's speak about theory... If an object (or what ever: information, etc) is teleported from A to B, you can't say that it travelled from A to B... I mean that it didn't follow the path from from A to B:
- at one moment, it was in A
- instentaneously, it disappear from A and appear in B (without moving)

To explain this, we can imagine that there are other dimension (then the 4 known) that connect the hole univers in a single point. Then, to go from A to B instantaneously, you just need to slide on one of these dimension (out of the space-time).... but this is just my opinion...

Very nice thought!!


So this means like the 4th dimension creating wormholes, doesn't have to be exactly the fourth, but let's take this one, aaah, and then the information, not being moved, but let's say Projected!! to that spot. Great!

a very good help lillo!

Or may be, but this might be far fetched, like the forth dimension creating a 'wormhole' let's say, this moving the information to a zero-point-dimension, let's say the fifth, hahahaa, that there being stored, or let's say, the fifth dimension having all information stored at all times and all spaces, but this going further,

like when an information from point A, wants to be delivered at point B. This point A, letting the fifth dimension now, this information wants to be revealed at point B, doing this by creating a wormhole through the 4th, touching the information in the 5th, which want to be seen at point B, and this locating a wormhole at point B letting the receiver accept the information from 'above'. WHahaaa! nutty professors. :p

But then again, would this work with material, projecting masses to other spots? The mass being slorped up in another dimension, let's say a zero-point dimension, this being when something is put in it, it can locate it anywhere else at speeds not seen, INSTANTLY! We're braking codes here

have a bite.

AA

hello,
sorry but going back to the second post on this thread, if light has a speed of 300,000 km/sec, and the earth, as we know has an oval orbit around the sun, and if im correct light from the sun takes about 8 mins to reach the earth (im not 100% on that).....does that mean light takes longer to reach the earth during winter than summer?

depends on if the planet is furthest from the sun in winter, then it would be correct to say yes. wouldn't it?

and it sounds logical that when an object is moving further from the sun, or a light source, that it takes longer for that light to reach it, offcourse, this in a matter of milliseconds. But unless stars, sometimes a star is allready burned out, and still light of it is seen on earth here. But with this star allready blown out.

for you cheese.

thanks for your reply alienaddicted,

i was assuming that the earth was futhest away from the sun during the winter, but thinking aout it i think i was totally wrong.

yes it seem obvious that light would take long to reach earth when it is futhest away from the sun, but i never really thought about it.

so at what point in the earths orbit of the sun is light measured to take 8 mins to reach the earth? or is this figure just 'rough' or a near estimate, or is it that even at the earths furthest point from the sun the difference in time too small to deviate from the 8 min estimate?

let's do some mathematics cheese.


So 8 minutes for the sunlight to reach the earth.

Light speeding through the universe at 300000km/s.

let's go


8 minutes is 8 x 60 seconds, like 480 seconds.

300000km/sec times 480 sec, for there are that much seconds within 8 minutes, if taken one second to travel 300000kms, over 480 seconds =
144000000 kilometers has been travelled.

that's one hell of a distance, isn't it.

12 x 12 x 1000000

just nice numbers yea

The orbit of earth is an ellipse: we are closer to the sun in winter, and further during the summer... Here is a nice tutorial about that daphne.palomar.edu...

The orbit of earth is an ellipse: we are closer to the sun in winter, and further during the summer... Here is a nice tutorial about that daphne.palomar.edu...

Depends on the hemisphere you are in. Better to state the month rather than the season... The site you gave says this:

The point where the Earth is closest to the sun is called perihelion (from the Greek peri, close or near, and helios, meaning sun). Perihelion takes place on January 3, which, of course, is during winter for the Northern Hemisphere, and summer for the Southern Hemisphere. The point where the Earth is farthest away from the sun is called aphelion (Greek ap, away from, and helios, sun). Aphelion takes place on July 4, which is winter for the Southern Hemisphere and summer for the Northern Hemispohere. So, actually the Earth and sun are closest during the Southern Hemisphere summer.

Cheers.


[edit on 12-1-2006 by d60944]

Originally posted by alienaddicted
But then again, would this work with material, projecting masses to other spots? The mass being slorped up in another dimension, let's say a zero-point dimension, this being when something is put in it, it can locate it anywhere else at speeds not seen, INSTANTLY! We're braking codes here

It depends from which dimension the masses belongs to... if the masses belongs to our 4 dim world, then 5 dim are enough... if the masses belong to the fifth of the sixth dim, then we need more then 5 dimensions...
Let make a comparison:
- Put a vector on plan: it designs a line.
- If you rotate the vector, it remains only a spot on the plan: so if you are watching this spot from the plan, you don't have enough information to design the vector again.
- Now if you watch the vector from a 3 D world, it can take any direction, you still can design it

We can make the same though with a triangle in a 3 D world: you need to have 2 point of view to define exactly the triangle. So you need to be able to move... and so you need 4 dimension

What about a volume?...... and what about a moving volume?

lillo masterbrain, I can't really figure it out...


braindamage might occure :s



hmz, to go further on d60944, depends in what hemisphere it is?

Do you mean one time in winter the earth is like on the oval edge of the elipse, and say 10 years later it is on the circular edge of it?

I don't really get it, or should I sleep more.


AA

I'm taking quantum physics right now, and already in two lectures we have had a lot of stuff shoved down our throats. The biggest obstacle right now is that you have to just accept for now that light has wave properties and particle properties. It is both the light wave, with a frequency and wavelength, as well as the photon, with velocity and mass.

However, photons are categorized as "virtual" masses or virtual particles. Mathematically they have mass just so we can make sense of things and predict how they will act, but in actuality do not have mass. Fun stuff huh?

I will try to post more specific stuff on here when I have the time (a few hours).

Originally posted by alienaddicted


12 x 12 x 1000000

That is a rough estimate of how long it should take light from the sun to reach earth assuming a set of favorable circumstnaces.

Originally posted by d60944

The orbit of earth is an ellipse: we are closer to the sun in winter, and further during the summer... Here is a nice tutorial about that daphne.palomar.edu...

Depends on the hemisphere you are in. Better to state the month rather than the season... The site you gave says this:

It really does not depend. How would it depend?

It really does not depend. How would it depend?

Ummmm, because winter in the northern hemisphere is at the same time as summer in the southern hemisphere, and vice versa.

Cheers.

ah, now I've got it, hemisphere, the spot on earth where you're at let's say.

I couldn't figure out what you've ment.

k

AA